replace "shape error across CALL" with more information.
x.go:7: not enough arguments to CALL
a int, b int
int
x.go:10: assignment count mismatch: 3 = 2
x.go:12: too many arguments to RETURN
[no arguments expected]
int, int, int
also leave type alone after conversion failure,
for later errors:
bug049.go:6: cannot convert nil constant to string
bug049.go:6: illegal types for operand: EQ
string
nil # this used to be blank